Paleomagnetism and Problem-Solving | September 5, 2012

With each challenge it encountered, the more the team seemed to take on a life of its own, emerging as a dynamic, complex organism, much more than the sum of its parts. Facing another technical hurdle, it continued to push toward its goal of mapping the underground faults and fractures in Surprise Valley relying on each other’s ideas and expertise. It quickly adapted to the new challenge, shifting plans as necessary. While I learned more about how geophysicists collect magnetic data on the ground, SIERRA engineers engaged in an intense brainstorming process, culminating in a potential breakthrough.

Yesterday morning I followed lead scientists Jonathan Glen and Noah Athens, both from the U.S. Geological Survey, out to the field, planning to observe them as they communicated with SIERRA while driving four-wheel all-terrain vehicles (ATVs). Not long after they unloaded the ATVs, Jonathan received a call from the ground base station. The crew had lost all signal from the UAS’ fluxgate magnetometer, which would allow them to correct for magnetic noise associated with the magnetization of the aircraft that could obscure signals arising from geologic structures they’re interested in mapping. After hanging up, Jonathan leapt back into his truck and sped off to Cedarville Airport to rejoin the team, leaving Noah and I to collect magnetic data by foot instead.

Lead scientist Jonathan Glen of the U.S. Geological Survey (USGS) drilling a basalt core in the Larkspur Hills

That afternoon, after Noah and I finished collecting magnetic data along the line the team had mapped, Jonathan picked us up in his truck, updating us on the crew’s troubleshooting session. The ground crew was still huddled beneath the hangar, intently trying to pinpoint the source of the communication problems. They thought that the aircraft vibrations had weakened a cable connection that prevented the fluxgate from transmitting compensation data to the ground, he explained as he drove us to an outcrop in the Larkspur Hills along the eastern rim of the valley.

There, Jonathan planned to drill cores from a dark, porous rock called basalt, which forms when lava cools. The team wants to determine the magnetic properties of the basalt at the surface, since this information could help them map the geologic features buried in the basin. We can’t determine the structure of the subsurface from magnetic field data alone, since magnetic fields aren’t unique; an infinite number of different structures can generate the same field.

But the team can use the magnetic properties of the basalt on the outcrop to model the features below the valley, assuming that the same basalt can be found in both areas. As you might recall from a previous post, tectonic forces have faulted and extended the crust across most of the western U.S. The result has been the development of a broad region of alternating basins and ranges. As magma broke through the crust in the Larkspur Hills millions of years ago, the lava flowed down to the valley below. Faulting continued and some of the basalt was buried under sediments in the valley, while some was uplifted and tilted and remains exposed at the surface.

The magnetic fields that leave their imprint on a rock over time collectively comprise the rock’s magnetization. Paleomagnetism parses this field into its composite parts. Assuming that the rocks on the outcrop are similar to those that are faulted and fractured below the basin, the team can feed the surface rocks’ magnetic properties into software they use to model the subsurface features. Each of the properties the researchers measure in their samples (i.e., density and magnetism) acts as a constraint, which helps to refine the model, so that it better represents the shape of the rocks in the subsurface.

Jonathan using an orienter to measure the spatial orientation of a basalt core

Geophysicists know that they’ve come up with a reasonable model when it’s consistent with geologic evidence and when the magnetic and gravity fields their model generates closely match the measured fields. They feed measurements from the rock samples into modeling software. The screen displays the magnetic and gravity readings as curves with various anomalies. The shape of these curves depends on the geometry and properties–that is, the density and magnetism–of the rocks in the subsurface. By constraining the density and magnetic properties of the model’s composite rock units to values within the range of the properties of the rocks they’ve sampled at the surface, the researchers are able to focus on determining the depth, shape, and extent of the rocks. Each value for these characteristics affects the curves’ spread and shape. Sometimes a value will improve the fit of one curve and not the other. When the model’s gravity and magnetic curves closely resemble the measured anomalies, then the geophysicists have produced a model that is consistent with the observed data. Any additional constraints, for example from seismic data or drill cores, that can be incorporated into the model, helps to further improve the likelihood that it accurately represents the subsurface.

Close-up view of an orienter

Geophysicists take paleomagnetic samples by drilling cores from rock using a modified chainsaw fitted with a cylindrical bit. Stopping at various rocks along the outcrop, Jonathan yanked hard on the pull cord, and drilled into the rock.

Jonathan and Noah then used an instrument, called an orienter, that will allows them to reconstruct the orientation of the core’s magnetization, measured in the lab, to its original orientation prior to sampling. With the core still embedded in the rock, Jonathan placed the orienter, into the hole he had just drilled and used it to measure the core’s angle (or azimuth) from North and its tilt with respect to the horizontal, which together uniquely define the core’s orientation in space. Jonathan then inserted a brass rod through a slot running lengthwise down the top of the orienter that left a brass mark on the core to indicate its top. Next he pulled out the orienter and extracted the core from the outcrop. He handed the core to Noah, who used a red felt-tip pen to draw marks to indicate the ends of the core that were on the inside and outside of the rock. Noah then slipped the core into a drawstring pouch for analysis in the lab.

But to accurately determine the core’s azimuth with respect to North, the researchers can’t always rely on a compass measurement. That’s because local magnetic sources can sometimes significantly deflect the compass needle.

To overcome this problem, scientists can use a solar compass to determine the core’s true angle from North. A long pin centered on the stage of the orienter casts a shadow across set of numbers encircling the edge of the instrument. The researchers record the angle of the shadow, which they then feed, along with the time, latitude, and longitude, into software that provides them with the solar azimuth, the angle of the sun from true north. From that value, they can obtain the true angle of the core with respect to North. The team also records the magnetic north azimuth, based just on a compass measurement, as a standard practice in case the sun isn’t visible. For example, the team might start orienting cores while the sun is still out, but by the time they reach the last few cores, the sky will have grown cloudy.

USGS researcher Noah Athens marking a basalt core’s spatial orientation within the rock from which it was drilled.

Jonathan knelt down before each rock he had drilled, taking measurements with the orienter and reading them off to Noah, who jotted them down in a pocket-sized notebook. It looked like laborious work, kneeling on the stone-strewn ground and squinting into the orienter as he took care to make accurate readings, the back of his neck red and glistening in the heat. We finished orienting the last sample just before the sun began slowly dipping below the horizon. After gathering our belongings, we drove back to the house, tired yet satisfied after a strenuous day of fieldwork.

At 10:30 that evening, one of the engineers for SIERRA’s scientific instrumentation, or payload, stopped by the house where the USGS team members were staying. He spoke excitedly about the ground base station’s troubleshooting session that day.

SIERRA engineers had fashioned custom test equipment and assembled a ground station to help ensure accurate troubleshooting. Puzzlingly, the UAS had no problem transmitting, or telemetering, the fluxgate data while the aircraft was grounded. The payload systems engineers painstakingly probed the scientific instrumentation for loose cables that might be preventing transmission specifically of the fluxgate compensation data. Meanwhile, the other SIERRA engineers evaluated complementary systems, like the electronics. They performed a complete electrical system analysis, meticulously testing each connector’s communication with the ground station.

Hunched over the payload, the payload systems engineers sounded positive and earnest as they shared their hypothesis about the aircraft vibrations with the rest of the team, although they suspected that something else was to blame. The others also sounded doubtful. Just to make sure, they simulated the aircraft’s vibrations by running SIERRA’s motors at high revolutions per minute (rpm). They didn’t observe any communication problems.

Why could SIERRA transmit compensation data from the ground but not from the air? The payload systems engineers suggested that the team try flying SIERRA again after sealing the loose cables in place with silicone. Even if the vibrations weren’t causing the communication problems, maybe securing the cables would still somehow solve the actual problem, whatever that might be.

“Unless you find the problem, I am not going to send the aircraft back up,” SIERRA lead engineer Randy Berthold said firmly.

Having already lost precious flight time, the team began heatedly batting around ideas. “Maybe the fluxgate just can’t transmit the data quick enough,” ventured SIERRA engineer Ric Kolyer. Though he couldn’t specify a cause for the lag, the others sensed that he was leading them on the right track.

“I know what the problem is!” Geometrics engineer Misha Tchernychev exclaimed a few minutes later. Geometrics is the company that manufactured the cesium vapor magnetometer and the replacement fluxgate. “It’s the baud rate.” Each instrument has a maximum rate of data transfer, or baud rate. The highest maximum baud rate possible with the cesium vapor magnetometer is 10 samples per second, or 10 Hertz (Hz). When the engineers installed the new fluxgate, which could sample more frequently, they decided to set its baud rate to 50 Hz to maximize the data it would collect per unit of distance traveled. But maybe the radio system they were using to telemetry the data to the ground station couldn’t handle such a high rate of data flow.

Ric came up with the analogy of pouring water through a funnel. The amount of water that can flow through a funnel is limited by the size of the funnel hole. If water is poured through the funnel faster than the funnel can drain it, the funnel will overflow. Likewise, the fluxgate was sending data to the radio faster than it could telemetry it to the ground station. The data overflowed, failing to reach the ground station.

“Everyone realized immediately that Misha was correct,” said Jonathan. “His diagnosis fit the symptom of the increasing loss of data…. It made perfect sense with the behavior of the instrument.”

The team reasoned that they didn’t have trouble communicating with SIERRA when it was grounded because they hadn’t allowed it to run long enough, leading them to trace the problem to flight vibrations. Using Ric’s analogy, water initially flows through a funnel, even when it’s poured too quickly. Only after the water is filled past the funnel does it start overflowing. Likewise, while SIERRA was aloft, the team initially saw magnetic data being transmitted via the radio link. They didn’t begin losing data until several minutes later. To test this theory, the team planned to ramp up the fluxgate’s maximum baud rate to cause the instrument’s communication with the ground station to fail. The fluxgate’s communication with the ground station should eventually fail, even with the aircraft landed.

The payload systems engineers and USGS researcher Geoff Phelps began steadily increasing the fluxgate’s data transfer rate. At the same time, the others continued a thorough test of the complementary systems. By then, dusk had already fallen. “Power up,” the payload systems engineers said with each rate increase, Geoff repeating after them as they turned on the aircraft. “Power down,” they said in succession as they shut it down. Though glassy-eyed and unshaven, they remained upbeat, feeling the solution just within grasp. As with the complementary systems, the other SIERRA engineers tested the communication of each of the fluxgate’s connectors with the ground.

At 10:00 that evening, convinced that they had definitively established the excessively high baud rate on the fluxgate as the problem, they dropped the maximum baud rate down to a manageable 10 Hz, enabling the instrument to sustain communication with the ground base station. Beaming, Randy agreed to let SIERRA fly again.

Tomorrow, the SIERRA will survey the north central detailed region and perimeter of Surprise Valley. If all goes well, it may even have time to survey another southern detailed region the following day in addition to the broad survey across the entire valley that the USGS researchers had originally planned. Though encouraged, the team members continue to hold their breaths. Stay tuned to find out whether tomorrow finds them back at the hangar or out on the tarmac, watching SIERRA complete its last few surveys of the valley.